Classification of four bar mechanism

Four bar mechanism, can be classified into two types.

• Class 1 four bar mechanism and
• Class 2 four bar mechanism

Lets the dimensions and name of the four links of the mechanism be as shown in the figure below.

Notation used in the discussion of classification of 4-bar mechanism

Here,
'a' denotes the largest link and its dimension;
'b' denotes the link opposite to 'a';
'c' is the largest among the two remaining links;
'd' denotes the link opposite to 'c'

Now, we can write,
a > b and c > d
Also (a+b) > (c+d)

The above established notation was done by Harding. Once we have understood this notation, we can classify any four bar mechanism as follows;

• Class 1 Mechanism : If the shortest of the four link can makes a complete 360 deg rotation, and all other links just oscillates, then that type of mechanism is called class 1 mechanism. Criteria for a mechanism to be class 1 is (a-b) < (c-d).
• Class 2 Mechanism : In this class, link dimensions are such that not any of the four links can makes a complete  360 deg rotation, criteria of mechanism to be class 2 is (a-b) > (c-d).

Various inversion of class 1 and class 2 mechanism can obtained which is discussed here